The accident occurred at 5:17 a.m. in the westbound lanes of the 10 Freeway at Indian Canyon in the Desert Hot Springs area. The big rig driver, hauling food products, had only moderate injuries and police said he reported feeling "a thump in the back, but that's all he really knew."

"The speed of the bus was so significant that when it hit the back of the big-rig trailer, the trailer itself entered about 15 feet into the bus," California Highway Patrol Border Division Chief Jim Abele said during a Sunday afternoon news conference. "So you can see there was a substantial impact."

All 44 people on board the bus were believed to be adults, said Abele, who wouldn't say if officials expected the death toll to rise.

"By the grace of God nobody else will pass away," he said.

The tour bus was identified as a 1996 USA Holiday bus. The Los Angeles-based company typically ferries people from LA to nearby casinos. The bus driver, who also owns the company and has not been identified, was killed in the crash.

Abele said the bus had been inspected in 2014, 2015 and April 2016 with no mechanical deficiencies reported. Records from the Federal Motor Carrier Safety Administration showed the bus had no prior crashes.

Authorities shut down the freeway in the aftermath of the incident. Police expected it to re-open at 4 p.m. local time.

Survivors told authorities that all passengers aboard the bus were believed to be asleep at the time of the crash. None of the passengers had been identified; however, Abele said police had been in contact with consulates from Mexico, Australia and Japan.

No cause of the crash was immediately identified, but Abele said many modern buses had "some kind of black box" that might aid authorities. Any influence of drugs, alcohol or driver fatigue had not been ruled out.

Firefighters on the scene had to use ladders placed near the bus' windows to pull out victims. It took nearly two hours to separate the vehicles, Fox News reported.

The Palm Springs hospital treated 14 patients, five in critical condition, three in serious condition and six with minor injuries. Palm Springs houses the valley's only trauma center, The Desert Sun Reported. All patients were adults.

Desert Regional director of marketing Rich Ramhoff said it was the most victims he'd seen from a single incident.

Eleven people with minor injuries were sent to Eisenhower Medical Center in Rancho Mirage, spokesperson Lee Rice said. JFK Medical Center received five patients, all with minor injuries, chief development officer Linda Evans said.